1. Field of the Invention
This invention relates to a fixable thermosensitive recording sheet, and more particularly it relates to a fixable thermosensitive recording sheet which has excellent shelf life before use for thermal recording and can develop color to a high density by thermal recording.
2. Description of the Prior Art
The recent years have seen amazing development in the field of information processing means such as high-speed printer, facsimile, etc., to meet the request of the society for outputting a vast volume of information as hard copies at as high speed as possible. A variety of techniques such as electrophotographing, electrostatic recording, electrosensitive recording, ink jet recording and thermosensitive recording are known as means for forming images on a recording medium according to the electric information in said information processing machines. Among these techniques, thermosensitive recording is rapidly gaining popularity in recent years owing to its relative simplicity of mechanism and relative uncostliness of recording sheet used therefor.
A thermosensitive recording sheet comprising a combination of a color forming material such as crystal violet lactone and a phenolic compound such as bisphenol A, which is used for said thermosensitive recording, is already known from, for example, Japanese Patent Publication No. 14039/1970. Such themosensitive recording sheet is widely used as copying paper for business matters and as recording paper for a variety of devices such as various types of recorders, electrocardiograph, desk computers, computer terminals, facsimile, etc.
These conventional means, however, involved the possibility that the printed letters could become illegible as a result of mistaken heating after printing to form color on the background or that the printed letters might be falsified, and thus the improvements on these matters have been strongly desired.
A thermo- and photo-sensitive material composed of a substance producing an alkali by thermal decomposition, a diazonium salt forming color with the alkali and a coupler compound, all layered on a substrate and, if necessary, further added with a binder, an acid stabilizer and/or other additives, is known as a diazo type sensitive material for thermal development. This diazo type sensitive material is advantageous in that it produces no irritant gas, as the development can be accomplished with heat alone, with no need of using ammonia gas which is used in the conventional dry type diazo method. Various types of color developers usable for such diazo type sensitive material have already been disclosed, and as typical developer compounds, there are known the inorganic or organic ammonium salts, urea or its derivatives, and salts obtained from neutralization with basic materials such as triethanolamine and heat-decomposable acids such as trichloroacetic acid. However, speed-up of such thermosensitive recording system, particularly elevation of the recording speed is desired lately, and it is required to create a sufficient color density with small heat energy.
In the method using a material, such as said above, which produces an alkali by thermal decomposition, it needs to use a material having a low thermal decomposition temperature for improving recording sensitivity. But, since such material undergoes thermal decomposition even at normal temperature, there may occur "pre-coupling" due to natural decomposition of the developer before development, resulting in thermal fogging and poor shelf life. On the other hand, use of a material with high thermal decomposition temperature for the improvement of shelf life may lead to an excess drop of color density as the thermosensitive recording speed is increased. In order to overcome such defects, in the field of copying sheet using a diazo type sensitive material for thermal development, it has been prepared to use as developer (heat-fusible developer) the fusible base particles which are theremally fused to produce an alkaline atmosphere, and as examples of such developer compounds, the aliphatic or alicyclic mono- or polyamines such as octadecylamine, hexadecylamine, decamethyleneamine, etc., are proposed in, for example, Japanese Patent Publication No. 1874/1964. These amines, however, are relatively hydrophilic, so that when a dispersion formed by ball milling and dispersing such compound with water is coated on a substrate after adding a diazonium salt, an azo coupler compound and other additives, the whole coating surface presents a slightly alkaline atmosphere due to said amine compound even under a non-heated condition, and this causes a reaction between the diazonium salt and the azo coupler compound to cause a slight degree of thermal fog to badly affect the shelf life, and further, the developed color density at the time of thermal development is also low. Moreover, when an acidic material is added to a particulate dispersion of the heat-fusible developer for improving the shelf life and the dispersion is coated on a substrate along with a diazonium salt and a coupler compound, there results excessively low density of developed color. There is also known a three-layer coating method in which the heat-fusible developer layer is separated from the diazonium compound-containing layer by interposing an intermediate layer, as for instance shown in Japanese Patent Publication No. 377/1965, for the purpose of improving the shelf life, but this method has the problem of elevated manufacturing cost and is also unable to provide a sufficiently high color density to meet the practical requirements in uses for high-speed thermosensitive recording as contemplated in this invention.
Japanese Patent Publication No. 40455/1976 discloses a diazo type thermally developed copying material obtained by dispersing a guanidine compound such as aminoguanidine, cyanoguanidine or 3-methoxybenzguanidine together with a water-soluble coupling component in a water-soluble sensitive solution composed of a photosensitive diazonium compound and a trichloroacetate (developer), then applying this dispersion on a substrate and drying same. But the guanidine compound used there is utterly different from the guanidine compound of this invention in both properties and function, particularly in that the former is soluble in water and that it is used as a material (eutectic material) for improving the coupling reaction rate (color developing rate) during thermal development by taking advantage of the fact that this material lowers the melting point of the water-soluble coupling component.